1. Field of the Invention
This invention relates to electrical switches of the press-to-actuate type. More particularly, it relates to the testing of electrical switches in a keyboard.
2. Description of the Related Art
Alpha-numeric keyboards of the type used for personal computers and the like can be tested using commercially-available pneumatic devices such as that described in U.S. Pat. No. 4,441,833 to Hasenbalg. In such devices, a pneumatic cylinders are connected to plungers which contact a particular keycap on the keyboard. Signals from the keyboard are monitored to determine whether a selected key has in fact been actuated when depressed by the corresponding plunger.
In Hasenbalg's apparatus, two pneumatic signals are required to actuate a particular plunger: a drive fluid; and, a latch release fluid. In this manner, a logical AND operation can be performed by each actuator and solenoid controlled valves can be employed in a matrix of "drive" columns and "latch release" rows, thereby minimizing the number of valves required in the system.
The piston in this type of apparatus acts against a spring which provides a return force to retract the plunger following actuation.
A disadvantage inherent in this type of tester is that the keys of the keyboard unit under test (UUT) are dynamically loaded. That is, the plunger contacts the keycap at a time when the piston (and its associated plunger) are in motion. Since any mass in motion has momentum, it is difficult to know or select the amount of force actually being experienced by the key switch as it is depressed by the plunger. A constant, easily-measured static force cannot be applied.
For example, it is often desired to test keyboards for sensitivity by applying a force to each keycap which should not cause switch actuation--e.g., the force applied by the fingers of a typist resting on the "home keys". This is a difficult test to accomplish using the type of apparatus described by Hasenbalg since if one selects a pneumatic pressure corresponding to the static force desired, that force will be exceeded when the extending plunger contacts the keycap owing to the momentum of the piston and plunger.
The present invention solves this problem and enables one to readily apply two different known forces to the keys of the unit under test. Most typically, these two forces will be that corresponding to a specified force which should not cause switch actuation and a different, higher force which is the minimum acceptable force for switch closure.